Nitinol alloys (also known as nitinol and shaped memory alloys), are shape recoverable alloys of nickel and titanium that, after either heating or cooling (as a function of the specific nitinol alloy under consideration) recover their original shape after the thermal process is reversed, i.e. heated metal is cooled or cooled metal is heated. These alloys are used in numerous critical medical applications. Their use normally is as a fine guide wire for interventional catheters, and for cardiac and peripheral stents to open blocked or partially blocked arteries, and related applications. All of these applications require fine cross sections of wire or tubing. In some cases the wire may be no more than ten to fifteen thousandths of an inch (0.0025 to 0.0038 cm) in diameter and the tubing wall similar or thinner thicknesses. Major limiting factors in the fatigue life of these devises are refractory inclusions in the metal. Refractory inclusions may be oxides, nitrides, carbides, metal oxy-nitrides and related compounds. These inclusions, while small, often are a large portion or percentage of the diameter of the fine wire or of the thickness of the thin walled tube used to make stents. These inclusions act as hard spots in the otherwise ductile alloy. While flexing in use in the heart or other in vitro sites, the softer metal flexes and can in effect break or fatigue around the hard inclusions in the metal.
These inclusions also render it difficult to fabricate the finely dimensioned wire or tube itself. Significant economic loss is incurred if fine wire or tube is drawn 90-95% of the way to its ultimate gage or thickness but cannot be finished due to inclusions. Further, fatigue failures due to inclusions or any reason, are not at all desirable when they occur in an alloy implanted in a patient
Inclusions are classically sourced to one of three origins. The first is surface or in solution gases found on or in the raw materials used to produce an alloy, in the case of nitinol, this would be nickel and titanium and to a lesser degree, chrome, niobium, copper, iron, platinum and other metals. These gases may be oxygen, nitrogen or carbon in elemental forms or otherwise tied up as some compound in the metal. The second possible source of inclusions is from contact with the refractory materials that contain the metals during melting. In the case of nitinol, this is almost exclusively graphite. Contact with graphite can generate carbide inclusions in the nitinol alloys.
The third source of inclusions in nitinol is from residual air left in the vacuum atmosphere of the melt furnace. This residual air, present as nitrogen gas, oxygen gas and perhaps a small amount of carbon, is available during the melt to react with the molten metal and to form deleterious oxides, carbides, metal oxy-nitrides, and nitrides. These in turn have the aforementioned affect as inclusions in metals, as hard spots, and fatigue limiting defects.
Efforts to date have been to reduce inclusions by techniques in primary ingot production, or the first melt of the alloy. These efforts include the use of the lowest oxygen and lowest nitrogen raw materials possible. They also include operating the melt furnace under the most rigorous vacuum regimes in order to preclude the introduction of gaseous elements left from the evacuated air.
To date, the net sum of all of the above efforts to reduce inclusions has been satisfactory for the state of the art as it is practiced today. Demands from the medical community to produce cleaner, more defect free nitinol alloys that will allow for longer fatigue lives of devices are continuing. Research is ongoing to develop cleaner alloys at reasonable prices. But, prior to the present invention the art had not found a method for reliably and consistently making high purity nitinol alloys with greatly reduced inclusion levels.
Nitinol ingots from which nitinol mill products are produced are melted in one of several ways.
The first melt technique is to assemble all of the alloy constituents and melt them more than one time and as many as five or six times in a Vacuum Arc Remelt (VAR) furnace. This technique produces useable product for current generations of nitinol devices. However, the repetitive melting allows two deleterious actions to occur. Each time the ingot is melted, slight additional amounts of oxygen and nitrogen are introduced into the ingot. These gaseous elements may occur in the alloy as oxides or nitrides or metal oxy-nitrides, or they may become solutes and occur as interstitial gases in the alloy. The former direct occurrence of oxides, nitrides and metal oxy-nitrides has already been established as undesirable. Recent work has demonstrated that the occurrence of gases as interstitial elements in a nitinol alloy allows the gases to be available to form or grow oxides or nitrides during the numerous hot working and annealing cycles necessary to produce final wire and tubing for use in implants.
The second melt technique is to melt the alloy constituents in a Vacuum Induction Furnace (VIM) using graphite crucibles. Then this primary VIM ingot is re-melted at least one time in a VAR to form a larger ingot. In addition to the inclusion forming mechanisms mentioned above in the first technique of multiple VAR melting, the VIM melting in graphite crucibles allows for the formation of many carbide inclusions in the alloy. When more than one final melt VAR ingot is used in association with this method, inclusions tend to agglomerate in the melt and become larger and therefore considerably more undesirable for the stated end applications.
The third method is to Induction Skull Melt in a water cooled copper induction heated crucible and produce an initial ingot. Like method two above, this ingot is then re-melted into a larger VAR ingot. This technique generates fewer or no graphite sic carbide inclusions as noted in method two. The inclusions found in ingots from method three are related more to raw materials and inadequate vacuum regimes.
In some cases a fourth method of directly processing as VIM melted ingots that were melted in graphite crucibles is also used. Small ingots are made using this technique. The technique allows for the introduction of multiple carbide inclusions.
Electroslag remelting (ESR), also known as ESR melting, was developed in the 1930s, but it took approximately 30 years before it became an acknowledged process for mass producing tool steels, superalloys and some nickel-based alloys. This process provides a chemically active slag that both protects a melt from contamination by constituents in the atmosphere and, may be effective to scrub, capture or otherwise remove, already existing inclusions in metal. In the case of critical rotating components for aircraft jet engines, certain specifications call for the primary melting of an alloy in a VIM, a subsequent re-melting of the alloy via ESR under protective slag designed to remove inclusions generated in the VIM, and then a final melting in VAR to develop the correct metallurgical structure to prepare the ingot for a subsequent hot working operations.
Because of the highly reactive nature of the titanium portion of the alloy ESR was never considered to be suitable for use in making NiTi alloys. Early development of nitinol alloys required the rigorous vacuum procedures already discussed. Growth in the nitinol industry followed already existing melting techniques, which did not include ESR. Subsequently, however, calcium fluoride (sometimes written CaF2 or CaFl) slag has been used to melt and weld other titanium alloys with significant success. Titanium is the most reactive component of the nitinol alloy. It contributes the vast majority of the inclusion forming constituents of the final alloy. Therefore, a concern for the high reactivity of titanium disproportionately addresses most of the concerns for inclusion generation in the alloy system. Nevertheless, those skilled in the art still did not consider ESR to be useful in melting nitinol alloys.